The present invention pertains to improvements in air leak detectors for use in the medical field. Many medical and surgical procedures require that the patient's blood be transported outside the patient's body, for example for connection to a heart-lung machine during surgery, or to an artificial kidney machine during hemodialysis. This extracorporeal transportation of blood is accomplished by means of blood tubing, with appropriate pumps, fittings, valves, etc. that may be required depending upon the particular procedure involved.
In the case of hemodialysis, the patient's blood is conveyed to the dialyzer by means of a length of tubing and a blood access device which connects the tubing to an artery. After purification by the dialyzer, the blood is returned through additional tubing to the patient. In addition to the dialyzer, the external blood flow path may include additional elements such as a pump and pressure monitors.
It will be appreciated that should an air leak develop anywhere in the external blood flow path, including the tubing, connectors, dialyzer, or heart-lung machine in the case of certain types of surgery, air would be allowed to enter the tubing. If a large air bubble were allowed to return with the blood to the patient's body, the result could be fatal. In order to prevent such occurrence, drip chambers are usually connected in the blood return path to the patient, to remove any air or other undissolved bubbles in the blood. In addition, good practice requires the use of some type of air leak detector, either on the drip chamber, on the blood tubing just prior to the return point to the patient's vein. It is also important that a clamping device working in conjunction with the sensor be provided for positively cutting off the flow upon detection of excessive air in the line.
Air leak detectors with spring-loaded clamps are disclosed in U.S. Pat. No. 3,935,876, issued Feb. 3, 1976, and assigned to the same assignee as the present invention. In that patent, air bubbles are detected and integrated, and a spring-loaded clamp is released to pinch off the blood tube when an excessive amount of air is detected.
In the preferred embodiment disclosed in U.S. Pat. No. 3,935,876, optical sensing is used for the detection of air bubbles, and some type of optical sensing has been used on most prior art air leak detectors, either on the blood tubing or on a drip chamber. One problem with optical sensing is that it is very difficult to get the light to penetrate more than a short distance into the blood, so that air bubbles passing through the center of a flow stream may not be detected. A more serious problem with optical sensing for drip chambers is the difficulty in distinguishing between blood or foam.
The possibility of accidental introduction of large quantities of blood foam is a major clinical problem in certain types of hemodialysis procedures. It is possible that air leaking into a blood line at a fistula connection prior to a pump would be converted into microfoam upon passing through a hollow fiber dialyzer. Microfoam can involve a substantial volume of air which is distributed in a tremendous number of extremely small air bubbles of less than 50 microliters each. This type of microfoam is essentially undetectable by any known optical sensing technique, and in fact may not even be visible to the eye of a careful observer.
Ultrasonic sensing has therefore been proposed for detecting air in drip chambers, since under favorable conditions, ultrasonic techniques can penetrate the blood, and can distinguish between blood and foam. However, the ultrasonic techniques which have heretofore been proposed have certain problem areas. One problem area is the difficulty of insuring good ultrasonic conduction between the plastic walls of the drip chamber and the transducers. In other types of ultrasonic measurement systems the transducers must be coated with an acoustically conducting gel, but this technique is undesirable in a clinical setting. Another problem area is the difficulty of maintaining accurate measurements in a system which attempts to measure pulse propagation time through the blood, due to the relatively small percentage change in propagation time caused by introduction of small air bubbles, and due to the difficulty of maintaining precise, repeatable spacing between sensors.